class User:

    def __init__(self):

        #another way is self.lista = {}
        self.lista = {}

    def addUser(self):

        newUser  = User()

        #rewrite list!
        newUser.lista = self.lista
        
        return newUser

    def __setitem__(self,el, va):
        self.lista[el] = va

    def __getitem__(self, el):
        return self.lista[el]

u = User()
u.lista = {'a':'b'}
print (u.lista)

u = u.addUser()
u[3] = 4
print (u.lista)


